When copying a map always set the descriptor array to describe the pre-allocated properties, even when descriptors are to be dropped.

Added a test which otherwise failed with an assert on ARM in debug mode. The reason for it only surfasing on ARM is that the NewObject runtime function is always used for allocating new JSObjects on ARM.

This change includes a few parts of http://codereview.chromium.org/174392 needed to trigger the error.
Review URL: http://codereview.chromium.org/173469

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2762 ce2b1a6d-e550-0410-aec6-3dcde31c8c00